This invention relates to an antireflection film which is used for exposure of a resist film in a fabrication process of a semiconductor device and an exposure-method in which an antireflection film is used.
In the field of semiconductor devices, together with the advancement of high integration of semiconductor devices, it has become a pressing need to establish a new process technique which permits working of very fine patterns of, for example, 65 nm or less. For the working of fine patterns, a photolithography technique is required essentially. The photolithography technique at present uses an argon-fluorine (ArF) excimer laser of the wavelength of 193 nm as an exposure light source in order to assure an enhanced optical resolution thanks to a reduced wavelength of exposure light (illumination light) so as to be ready for very fine working.
A silicon semiconductor substrate is patterned usually using a photosensitive resist layer formed by coating on the surface of the silicon semiconductor substrate. However, where the reflectance of exposure light (illumination light) at the interface between the resist layer and the silicon semiconductor substrate which serves as a substrate for the resist layer is high, a standing wave is induced conspicuously in the resist layer. As a result, side faces of the resist layer patterned by development exhibit a concave and convex shape in accordance with the shape of the standing wave. Therefore, there is a problem that a pattern of a good rectangular shape cannot be formed on the resist layer. It is to be noted that a pattern formed on a resist layer is sometimes called resist pattern. For example, the reflectance where a resist layer having a refractive index of 1.70 is formed on the surface of a silicon semiconductor substrate using exposure light of a wavelength of 193 nm is a very high value of approximately 60% where the exposure light is incident perpendicularly to the resist layer.
In order to solve such a problem as described above, according to a conventional photolithography technique wherein exposure light of a wavelength of 193 nm is used, an antireflection film of a single layer is formed between the silicon semiconductor substrate and the resist layer. For example, where the complex refractive index is represented by N0 (where N0=n0-k0i), if an antireflection film of a thickness of 100 nm having such values as n0=1.75 and k0=0.30 is formed on the silicon semiconductor substrate and a resist layer having a refractive index of 1.70 is formed on the antireflection film, then the reflectance is reduced by a great amount to approximately 2% where the exposure light is incident perpendicularly to the resist layer.
A related technique is disclosed, for example, in Japanese Patent Laid-open No. 2001-242630 (hereinafter referred to as Patent Document 1) or Boontarika, Ozawa and Someya, Extended Abstracts (The 65th Meeting2004); The Japan Society of Applied Physics, 2p-R-9 (hereinafter referred to as Non-Patent Document 1).